use a sync.Pool for ackhandler.Frames (#3656)

internal/ackhandler/frame.go

@@ -1,9 +1,29 @@
 package ackhandler
 
-import "github.com/lucas-clemente/quic-go/internal/wire"
+import (
+	"sync"
+
+	"github.com/lucas-clemente/quic-go/internal/wire"
+)
 
 type Frame struct {
 	wire.Frame // nil if the frame has already been acknowledged in another packet
 	OnLost     func(wire.Frame)
 	OnAcked    func(wire.Frame)
 }
+
+var framePool = sync.Pool{New: func() any { return &Frame{} }}
+
+func GetFrame() *Frame {
+	f := framePool.Get().(*Frame)
+	f.OnLost = nil
+	f.OnAcked = nil
+	return f
+}
+
+func putFrame(f *Frame) {
+	f.Frame = nil
+	f.OnLost = nil
+	f.OnAcked = nil
+	framePool.Put(f)
+}